With the advance of the gas turbine engine technology, there has been recognized a need for lightweight materials which can resist deterioration at high temperatures and have sufficient mechanical properties to withstand strenuous operating conditions. The metallurgical art has described a wide variety of superalloys developed for that purpose. Frequently, such superalloys are based on nickel and preferably are in the form of single crystal articles for such gas turbine components as turbine airfoils. Also, effort has been directed to the development of high temperature alloys based on cobalt or iron.
Intermetallics of Ni and Al have been the subject of investigations as replacements for the superalloys currently used in gas turbine engines. Many such investigations have been directed to improvements and refinements in Ni.sub.3 Al. More recently, however, interest has been exhibited in connection with intermetallic compounds such as those based on the NiAl system because of their relative lower density along with the potential to be used at high temperatures, for example, as a turbine airfoil. Compared with nickel base superalloys, their density can be up to about 33% lower, and their thermal conductivity can be up to about 300% higher. However, the low ductility of binary NiAl intermetallics, less than 1% between room temperature and about 600.degree. F., had impeded the implementation of NiAl intermetallics as a viable substitute for nickel base superalloys. More recent efforts to improve ductility in such compounds are described in U.S. Pat. Nos. 5,116,438; 5,116,691; and 5,215,831--Darolia et al, assigned to the assignee of the present invention. Those patents include extensive background and description of efforts in connection with the NiAl intermetallic system and their disclosures are hereby incorporated herein by reference to be a part of this background presentation. Of particular interest to the preferred form of the present invention is the U.S. Pat. No. 5,116,438 describing the microalloying of the NiAl system with gallium to significantly improve the low temperature ductility of the system. Resulting from such alloying is a microstructure characterized by a more ductile single phase matrix. Reference to the phase diagram for the NiAl intermetallic shows that from about 45 at % to about 59 at % Ni with the balance Al, that intermetallic exists as a single beta phase. That phase exists up to its melting point in the range of about 2950.degree.-3000.degree. F.
Such an intermetallic alloy can be useful for selected applications not requiring the high temperature strength needed in hot turbine engine components. However, those alloys do not possess adequate high temperature strength to be competitive with the more advanced nickel base superalloys. Nevertheless, the NiAl system is very attractive for use as turbine blading members because their lower density, and associated weight reduction, and their higher thermal conductivity, and associated more effective cooling of the component, can result in more efficient engine operation. The stresses in NiAl intermetallic alloy airfoils can be significantly lower than in superalloy blades under the same operating conditions. Therefore, development of a NiAl intermetallic alloy with improved high temperature mechanical strength properties, along with good low temperature ductility to enable manufacture and initiation of operation, is highly desirable.